I am often surprised at the number of people who are not aware of the Genealogical Society of Utah (GSU) and their Family History Library (FHL): www.familysearch.org Perhaps this is because this is a non-profit organization, with different funding and goals than commercial organizations. The point being, if you are working on your Irish or Irish immigrant genealogy, you need to be aware of what the GSU has microfilmed, digitized or indexed. The GSU has amassed the largest collection of Irish related records outside of Ireland and Northern Ireland.
This brings me to the point about the Family History Library Catalog (FHLC). The catalog on the website can be so massive that you have to sometimes outguess what the cataloger so you can find a record. Much of this is due to the hodge-podge nature of many Irish collections.
All counties are listed as “Ireland.” To avoid confusion, the FHL chooses old boundaries which make sense to researchers. They do this with the old counties in the UK as well as for Germany.
The catalog is divided into levels, and sometimes it’s easier to check them all rather than. Much depends on what you know. If you know a county already, then try that as a category. If you know a parish (civil not Catholic), then try that as a category. A town will be a category, such as Belfast or Dublin, but unless a record is specific to a townland, those will not be listed. Remember, much is hidden under general “Ireland.”
The civil parish has been chosen as a basic unit for cataloging local records. So to find a Catholic parish record, you will need to make sure you convert the boundaries. I use Brian Mitchell’s A New Genealogical Atlas of Ireland (2002) to do the job. Then you can use the FHLC more accurately.